This application generally relates to electrochemical storage devices, more particularly to the application of graphitic nanofibers as electrodes in electrochemical capacitors.
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
Electrochemical capacitors (ECs) are gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. Compared with conventional capacitors, ECs have extremely high capacitance values, limited frequency response, high equivalent series resistance (ESR) which is directly related to electrode thickness and inversely proportional to the cross sectional area of the electrode, voltage-dependent capacitance, and voltage-dependent self-discharge rate. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. And in electric vehicle applications, large ECs can xe2x80x9cload-levelxe2x80x9d the power on the battery system and thereby increase battery life and extend vehicle range.
Capacitors store energy in the electric field between two oppositely charged parallel plates, which are separated by an insulator. The amount of energy a capacitor can store increases as the area of conducting plates increases, the distance between the plates decreases, and the dielectric constant (the ability to store charge between the plates) of the insulating material increases.
ECs are distinguishable from traditional electrolytic capacitors which store energy by charge separation across a thin insulating oxide film that is often formed by a controlled electrolytic oxidation process at an appropriate metal.
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective xe2x80x9cplate areaxe2x80x9d and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1 nm, thus forming an extremely small effective xe2x80x9cplate separationxe2x80x9d. In some ECs, stored energy is substantially augmented by so-called xe2x80x9cpseudocapacitancexe2x80x9d effects, occurring again at the solid-electrolyte interface. Double layer capacitances are commonly of the order of 16-40 xcexcF cmxe2x88x922 while pseudocapacitances associated with EC systems are commonly 10-100 xcexcF cmxe2x88x922.
The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at each electrode providing double-layer capacitance. The carbon provides a high surface area, A, and the effective d is reduced to an atomic scale, thus providing a high capacitance.
Although the energy storage capability of the double layer was recognized more than 100 years ago, it took the development of low-current-draw volatile computer memories to create a market for ECs.
Conventional electrochemical energy storage is achieved in a galvanic cell or a battery of such cells. The energy corresponds to the charge associated with chemical redox changes that can occur in the battery on discharge, multiplied by the voltage difference between the electrodes of the cell. The discharge process involves a net chemical reaction in the cell associated with passage of a certain number of electron or faradays per mole of reactants.
If an electrochemical reaction, such as a redox process, should occur at or near the electrode, the capacitance may be further increased. This increased capacitance is sometimes termed xe2x80x9cpseudocapacitancexe2x80x9d and the resulting device, while properly an electrochemical capacitor, is informally called a pseudocapacitor, supercapacitor or ultracapacitor. An electrochemical capacitor will have a different cyclic voltammogram than a pure double-layer capacitor, the pseudocapacitance revealing a Faradaic signature.
Redox systems, especially of RuO2.xH2O, for electrochemical capacitors have been demonstrated (Zheng, Z. P. and Jow, T. R., xe2x80x9cA new charge storage mechanism for Electrochemical Capacitorsxe2x80x9d, J. Electrochem. Soc., 142, L6 (1995)), but high cost and limited cycle life are continuing impediments to commercial use of such materials. The greater the Faradaic component of the capacitance, the more the discharge curves and life approach those of a battery rather than those of a capacitor. On the other hand, the specific goals of obtaining high power output suitable for EV applications cannot be met by a pure double layer capacitor using known or proposed electrode materials (Eisenmann, E. T., xe2x80x9cDesign Rules and Reality Check for Carbon-Based Ultracapacitorsxe2x80x9d, SAND95-0671xe2x80xa2UC-400 April 1995).
ECs do not approach the energy density of batteries. For a given applied voltage, capacitatively storage energy associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. This difference is due to the fact that in an ideal battery reaction, involving two-phase systems, charge can be accumulated at constant potential while, for a capacitor, charge must be passed into the capacitor where voltage and charge is being continuously built up. This is why energy storage by a capacitor is half that for the same charge and voltage in battery energy storage under otherwise identical and ideal conditions.
Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no xe2x80x9cmemory effectxe2x80x9d, and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
However, readily available EC products are limited in size and power performance, due primarily to their targeted memory backup use. They have capacitance values of up to a few Farads, an equivalent series resistance (ESR) of one to fifty ohms, and a working voltage of 3 to 11 V.
Until recently, ECs suitable for high-power applications have been unavailable. But interest in automotive starting, lighting and ignition (SLI) applications, as well as in electric vehicle (EV) load-leveling, has stimulated product development activities for such high-power devices. The goal is to develop products that can be efficiently charged and then discharged in the time specified for these high-rate applications.
Severe demands are placed on the energy storage system used in an EV. The system must store sufficient energy to provide an acceptable driving range. It must have adequate power to provide acceptable driving performance, notably acceleration rate. In addition, the system must be durable to give years of reliable operation. And finally, the system must be affordable. These four requirements are often in conflict for candidate energy storage technologies. This situation creates significant challenges to developers of EV energy storage systems.
A capacitor offers significant advantages to the EV energy storage system. But to be useful, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, provide high cycle-life ( greater than 100,000 cycles), and meet specified volume, weight and cost constraints. This capacitor does not exist presently.
Electrochemical capacitors, sometimes called ultracapacitors, or supercapacitors, are of interest in hybrid electric vehicles where they can supplement a battery used in electric cars to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be used for cruising, but capacitors (because they release energy much more quickly than batteries) would kick in whenever the car needs to accelerate for merging, passing, emergency maneuvers, and hill climbing. To be cost and weight effective compared to additional battery capacity they must combine adequate specific energy and specific power with long cycle life and meet cost targets, as well.
The performance characteristics of an electrochemical capacitor are fundamentally determined by the electrochemistry of the electrodes. Many previously proposed electrode materials result in unacceptably high cost capacitors.
The energy stored in a charged capacitor can be continuously increased in proportion to the increase of the voltage, limited only by electrical breakdown of the dielectric. The maximum available stored energy, for a given chemical species, is determined by the quantity of electroactive materials, their standard electrode potentials and their equivalent weights, and the power by the reversibility of the electrochemical changes that take place over discharge together with the electrical resistivity of the materials and external circuity.
Experience with carbon electrode electrochemical capacitors shows that geometrical capacitance calculated from the measured surface area and the width of the dipole layer is not routinely achieved. In fact, for very high surface area carbons, typically only about ten percent of the xe2x80x9ctheoreticalxe2x80x9d capacitance seems to be found.
This disappointing performance is related to the presence of micropores and ascribed to wetting deficiencies and/or the inability of a double layer to form successfully in pores in which the oppositely charged surfaces are less than about 20 xc3x85 apart. In activated carbons, depending on the source of the carbon and the heat treatment temperature, a surprising amount of surface can be in the form of such micropores (Byrne, J. F. and Marsh, H., xe2x80x9cIntroductory Overviewxe2x80x9d in Patrick, J. W., Porosity in Carbons: Characterization and Applications, Halsted, 1995).
The fundamental reasons for considering electrochemical capacitors instead of batteries are power density and life. While these are inherent in a truly capacitative system, the energy density goals sought in, for example, electric vehicle applications cannot be met in such a system. Efforts to push the envelope of electrochemical capacitor energy storage always rely on a substantial fraction (most) of the capacitance coming from a Faradaic mechanism.
It would be desirable to produce a electrochemical capacitor exhibiting greater geometrical capacitance using a carbon based electrode having a high accessible surface area, high porosity, and reduced or no micropores. The presence of micropores in the current carbon based electrodes makes it inapplicable to the EV energy storage system.
It is therefore an object of this invention to provide a carbon nanofiber based electrode to increase the performance of the electrochemical capacitor.
It is also an object of this invention to surface treat the nanofiber based electrode to modify the Faradaic capacitance.
It is a further object of this invention to provide a functionalized nanofibers for use in such electrode.
In its broadest embodiment, the invention relates to the application of a nanotube (nanofiber) for electrochemical capacitor electrodes as shown in FIG. 1. In particular, said nanofiber is a graphitic nanofiber. More particularly, the invention relates to electrodes comprising carbon nanofibers having high surface area ( greater than 100 m2/gm) and being substantially free of micropores (i.e., pores with diameter or cross-section less than 2 nm). Even more particularly, the invention relates to the use of such electrode in electrochemical capacitors. Preferably, the electrodes each comprise carbon nanofibers functionalized with different functional group.
In one embodiment, the invention relates to a capacitor having an electrode comprising nanofibers having a surface area greater than about 100 m2/gm. Advantageously, the nanofibers are substantially free of micropores or the micropores contribute not more than 5% of the surface area.
Preferably the nanofibers are functionalized, for example with one or more functional groups selected from quinone, hydroquinone, quaternized aromatic amines, mercaptans or disulfides. The functional groups may be contained in a ladder polymer of the formula 
wherein G is CH or N,
or may be a graphenic analogue of one or more of 
The nanofibers are advantageously carbon nanofibers being substantially cylindrical with a substantially constant diameter, having graphitic layers concentric with the nanofiber axis and being substantially free of pyrolytically deposited carbon.
The nanofibers may be coated with a thin coating layer of a pyrolyzed carbonaceous polymer. The coating layer preferably comprises one or more polymers selected from the group consisting of phenolics-formaldehyde, polyacrylonitrile, styrene DVB, cellulosic polymers, and H-resin.
The nanofibers may be intertwined and interconnected to form a rigid porous carbon structure.
In another embodiment the invention relates to an electrode comprising nanofibers having a surface area greater than about 100 m2/gm. Advantageously, the nanofibers are substantially free of micropores or the micropores contribute not more than 5% of the surface area.
Preferably the nanofibers are functionalized, for example with one or more functional groups selected from quinone, hydroquinone, quaternized aromatic amines, mercaptans or disulfides. The functional groups may be contained in a ladder polymer of the formula 
wherein G is CH or N,
or may be a graphenic analogue of one or more of 
The nanofibers are advantageously carbon nanofibers being substantially cylindrical with a substantially constant diameter, having graphitic layers concentric with the nanofiber axis and being substantially free of pyrolytically deposited carbon.
The nanofibers may be coated with a thin coating layer of a pyrolyzed carbonaceous polymer. The coating layer preferably comprises one or more polymers selected from the group consisting of phenolics-formaldehyde, polyacrylonitrile, styrene DVB, cellulosic polymers, and H-resin.
The nanofibers may be intertwined and interconnected to form a rigid porous carbon structure.
In a further embodiment the invention relates to a capacitor, comprising:
a first electrode, the first electrode comprising nanofibers having a surface area greater than about 100 m2/gm;
a second electrode, the second electrode comprising nanofibers having a surface area greater than about 100 m2/gm; and
an electrode separator disposed between the electrodes, the electrode separator comprising an electrically nonconductive and ionically conductive material.
Advantageously, the nanofibers are substantially free of micropores or the micropores contribute not more than 5% of the surface area.
The nanofibers of the electrodes are functionalized, for example with one or more functional groups selected from quinone, hydroquinone, quaternized aromatic amines, mercaptans or disulfides. The functional groups may be contained in a ladder polymer of the formula 
wherein G is CH or N,
or may be a graphenic analogue of one or more of 
The nanofibers of said first electrode may be functionalized with the same functional groups as said nanofibers of said second electrode.
The nanofibers of said first and second electrodes are advantageously carbon nanofibers being substantially cylindrical with a substantially constant diameter, having graphitic layers concentric with the nanofiber axis and being substantially free of pyrolytically deposited carbon.
The nanofibers of said first and second electrodes may be coated with a thin coating layer of a pyrolyzed carbonaceous polymer. The coating layer preferably comprises one or more polymers selected from the group consisting of phenolics-formaldehyde, polyacrylonitrile, styrene DVB, cellulosic polymers, and H-resin.
The nanofibers of said first and second electrodes may be intertwined and interconnected to form a rigid porous carbon structure.
The nanofibers of said first electrode may be functionalized with different functional groups than said nanofibers of said second electrode.
The second electrode advantageously has a redox voltage different from said first electrode. Preferably the redox voltage of said second electrode is near zero (NHE).
In yet another embodiment the invention relates to a capacitor comprising a plurality of cells,
each cell comprising:
a first electrode, the first electrode comprising nanofibers having a surface area greater than about 100 m2/gm;
a second electrode, the second electrode comprising nanofibers having a surface area greater than about 100 m2/gm; and
an electrode separator disposed between the electrodes, the electrode separator comprising an electrically nonconductive and ionically conductive material; and
an electrical connection between the cells.
In still another embodiment the invention relates to a wound roll capacitor comprising two layers of nanofiber electrode having a surface area greater than about 100 m2/gm and at least two layers of electrode separator comprising electrically nonconductive and ionically conductive material.
The invention also relates to a capacitor, comprising
a first electrode having a surface area greater than about 100 m2/gm;
a second electrode having a surface area greater than about 100 m2/gm, wherein the second electrode has a redox potential different from said first electrode;
an electrode separator disposed between the electrodes, the electrode separator comprising an electrically nonconductive and ionically conductive material; and
wherein redox reactions occur only at the surface of said first and second electrodes.